If a mobile station is in an idle state and moves across different Radio Access Technologies (RATs) coverage areas, inter-RAT idle hand-off is conducted. In order to ensure that a channel condition of a target carrier having a different RAT type than a serving carrier is sufficient, and in order to provide enough information for an access network to determine a final inter-RAT idle hand-off, idle hand-off pre-preparation is required. The idle hand-off pre-preparation may include synchronizing the mobile station with the target carrier, measuring/reporting a pilot power of the target carrier, pre-checking in to the serving carrier, and pre-registration upon exiting a border area.
Triggering the idle hand-off pre-preparation and the final idle hand-off are still open issues in the field of inter-RAT idle hand-off. In a conventional single technology system, the idle hand-off is normally triggered by a power measurement and the idle hand-off generally occurs at a sub-net boundary. If there are sharp boundaries between the subnets and a mobile station travels along a sub-net boundary area, a ping-pong issue occurs because the mobile may travel back and forth across the boundary which leads to un-necessary, frequent registrations at both sides of the sub-net. A conventional solution to the ping-pong issue is to use a hysteresis of the power measurement. However, the hysteresis of the power measurement is not reliable due to shadowing and fading in a mobile environment. The hysteresis of the power measurement is also performed at the expense of power efficiency.
Another conventional method provides a secondary color code method which uses spatial hysteresis to resolve the ping-pong issue. The secondary color code method provides a more efficient method to resolve the ping-pong issue at the sub-net boundary, but only for intra-RAT idle hand-off, and the secondary color code method introduces complexity on requesting cross sub-net paging. The secondary color code method only triggers a mobile to register and start the idle hand-off if the mobile exits the border area.
Conventional methods conduct the inter-RAT idle hand-off or the inter-RAT handoff in the connected state based on a forward link signal-to-noise ratio measurement. In many cases, a current mechanism for measuring an Ec/l0 signal-to-noise ratio of a serving sector's pilot as a basis for determining when to perform a hand-off to another frequency or technology does not trigger at the right time or place if the mobile station is moving off the edge of a border sector and the forward link is not interference limited. Accordingly, the mobile station measures reasonable forward link Ec/l0 signal-to-noise ratio values in locations where the reverse link is broken, because interference in the forward link may come from adjacent cells. However, for the cells at the coverage edge there is much less interference due to a smaller number of adjacent cells to the edge cells. Accordingly, even if a mobile station is far away from the base station, the forward link signal to noise ratio is still relatively good. Therefore, in this scenario both the actual signal power and the interference are lower in the forward link. As such, the mobile station is unreachable and cannot originate, even if the forward link signal to noise ratio measurement suggests that the mobile station could have been operating properly.